Method for recovering high grade process energy from a contact sulfuric acid process

ABSTRACT

An improved process for the recovery of high grade energy from a contact sulfuric acid manufacturing process. Improvements include: injection of steam between an intermediate catalyst stage and a heat recovery absorption tower and/or a heat exchanger for transfer of heat from conversion gas to high pressure boiler feed water; use of a condensing economizer for recovery of the vapor phase energy of formation of sulfuric acid from a wet conversion gas; and use of heat recovery system absorption acid for preheating air to a sulfur burner, the heat transferred to the combustion air being recovered at high pressure and temperature in a waste heat boiler.

BACKGROUND OF THE INVENTION

This invention is directed to various improvements by which high gradeenergy is recovered in the contact process for the manufacture ofsulfuric acid. More particularly, the invention is directed toimprovements in which the energy of low pressure steam is recovered athigh temperature, the vapor phase heat of formation of sulfuric acid inwet conversion gas is recovered in an economizer, and absorption heat isused to preheat oxygen-containing gas used for combustion of a source ofsulfur in generation of sulfur dioxide to be fed to the converter. Theenergy recovered in the latter step is upgraded by transfer of heat fromthe sulfur dioxide combustion gas to a high temperature heat transferfluid in a waste heat recovery unit.

Until recently, only about 55% to 60% of the heat generated in thecontact sulfuric acid process was recovered in useful form. A majorimprovement in energy recovery has been provided in the processes ofMcAlister and Ziebold U.S. Pat. Nos. 4,576,813 and 4,670,242, andMcAlister copending application Ser. No. 369,301, filed June 21, 1989,all of which describe processes for the recovery of the heat ofabsorption in the form of medium pressure steam. In the heat recoverysystem described in these disclosures, an absorption tower is operatedat high temperature and heat is transferred from the absorption acid toproduce medium pressure steam. By maintaining the acid concentration inthe range typically of 99% to 100%, alloy heat exchangers may be usedfor recovery of the absorption heat. Practice of the McAlister andZiebold processes allows process heat energy recovery capability to beincreased to the range of 90 to 95%. The process of pending Ser. No.369,301, which is expressly incorporated herein by reference, describesthe particular application of absorption heat recovery to a process inwhich sulfuric acid is produced from a wet sulfur dioxide-containinggas.

SUMMARY OF THE INVENTION

A central object of the present invention is the provision of animproved process in which process energy is recovered from a contactsulfuric acid manufacturing process in high grade form. More particularobjects of the present invention include the recovery at hightemperature of the heat generated by vapor phase formation of sulfuricacid in a wet conversion gas; the upgrading of absorption energyrecovered in an absorption heat recovery zone; and the upgrading of lowpressure steam by transfer of the energy contained in the steam to ahigher temperature heat transfer fluid.

Briefly, therefore, the present invention is directed to an improvementin a process for the manufacture of sulfuric acid. The process comprisescombustion of a source of sulfur and an oxygen-containing gas in aburner to produce a combustion gas stream comprising sulfur dioxide andoxygen, passage of the gas stream through a plurality of catalyst stagesfor progressive conversion of sulfur dioxide to sulfur trioxide,recovery of heat in useful form by cooling the gas stream exiting thecatalyst stages, passage of the cooled gas stream from one of the stagesthrough an absorption zone where the gas stream is contacted withsulfuric acid for removal of sulfur trioxide from the gas phase, andreturn of the gas stream from the absorption zone to a further stage ofthe plurality of catalyst stages. The improvement comprises introducingwater vapor into the gas stream at a point between the burner and theabsorption zone. At least a portion of the water vapor reacts withsulfur trioxide in the gas phase to produce sulfuric acid and therebygenerate the heat of formation of sulfuric acid in the gas phase. Heatenergy equivalent to at least a portion of the gas phase heat offormation of sulfuric acid is recovered from the gas phase, the heatbeing recovered in steam having a pressure at least about 2.5 bar higherthan the pressure of the water vapor as introduced into the gas stream.

The invention is further directed to an improvement in a process of thetype generally described above in which the gas stream that has exitedfrom said one stage contains vapor phase sulfuric acid that has beenformed by the reaction of water vapor and sulfur trioxide in the gasphase. The improvement comprises cooling the gas stream containing vaporphase sulfuric acid by transfer of heat to a heat transfer fluid in aneconomizer. The economizer comprises an indirect heat exchangercomprising heat transfer wall means between the gas stream and the heattransfer fluid. At least a portion of the wall means on the gas streamside thereof is at a temperature below the dew point of the gas streamentering the heat exchanger. Sulfuric acid thereby condenses from thegas stream on the wall means.

The invention is further directed to an improvement in a process for themanufacture of sulfuric acid, the process comprising combustion of asource of sulfur with an oxygen-containing gas in a burner to produce acombustion gas stream comprising sulfur dioxide and oxygen, recovery ofa portion of the heat of combustion by transfer of heat from thecombustion gas to a heat transfer fluid, catalytic oxidation of sulfurdioxide contained in the gas to produce a conversion gas containingsulfur trioxide, absorption of components of the conversion gas insulfuric acid in a heat recovery absorption zone, and recovery of atleast a portion of the heat of absorption from the absorption aciddischarged from the absorption zone. The improvement comprises heatingthe oxygen-containing gas in an indirect heat exchanger comprising apreheater for the burner, thereby contributing heat to the combustiongas. The oxygen-containing gas is heated with heat transferred from theabsorption acid discharged from the heat recovery absorption zone. Steamis generated at a pressure of at least about 25 bar by transfer of heatfrom the combustion gas.

Other objects and features will be in part apparent and part pointed outhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet illustrating a preferred process embodying theimprovements of the present invention;

FIG. 2 is a plot showing the vapor phase reaction equilibrium as afunction of temperature for the reaction of sulfur trioxide and water toform sulfuric acid in the vapor phase;

FIG. 3 is a plot showing sulfuric acid condensate composition as afunction of the ratio of equivalent water vapor to equivalent sulfurtrioxide in the gas phase, for example, after steam injection, thedotted curve showing the composition of the first drop of condensationand the solid curve showing the composition after 20% of condensablecomponents have been condensed; and

FIG. 4 contains plots of sulfuric acid mist formation in the heatrecovery absorption zone as a function of temperature and equivalentwater/sulfur trioxide ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, significant enhancements areachieved in the form in which energy is recovered from a contactsulfuric acid process. These improvements are adapted to provide furtherimprovement in the heat recovery processes described in the abovementioned patents of McAlister and Ziebold, and in the pendingapplication of McAlister. They are particularly adapted for use in a wetgas contact process of the type described in the latter application.

It will be understood that each of these improvements may also findapplication in processes in which dry sulfur dioxide-containing gas issupplied to the sulfur trioxide converter. Moreover, regardless ofwhether absorption heat recovery technology is used, injection of watervapor into the conversion gas can be used to recover the vapor phaseheat of formation of sulfuric acid at high temperature, and a condensingeconomizer can be used to maximize the amount of high grade energyrecovered from a wet conversion gas. For example, it may be advantageousto apply these improvements to an existing plant that does not havefacilities for implementation of absorption heat recovery technology.However, the maximum benefit of the process of the invention is realizedin an absorption heat recovery plant of the type illustrated in the FIG.1.

A number of advantages result from elimination of the need for a dryingtower by operation of a wet gas system. Avoided is a substantialinvestment otherwise required for the drying tower itself, andassociated circulating pump, piping, pump tank, and cooler. Also avoidedis the need for cross flow from the drying tower to the heat recoverytower to transfer water accumulated in the drying operation. There is aconsequent reduction in the volume of acid leaving the heat recoverytower acid circuit, which is advantageous because it is difficult torecover in high grade form all the heat contained in the acid flowingout of the heat recovery tower circulating loop. Concentration controlis generally simplified throughout the process. Energy recovery isincreased because the heat of condensation of water vapor, which isnormally removed in the drying tower cooler and lost, is shifted to theheat recovery system where it is recovered in the form of mediumpressure steam.

FIG. 1 depicts the flow sheet for a wet gas acid plant in whichelemental sulfur or a source thereof is burned with undried ambient airin a sulfur burner 101 to produce a combustion gas containing sulfurdioxide and oxygen. Alternatively, a sulfur dioxide stream may bederived from such sources as the roasting step of a metal recoveryoperation, the reference herein to burning or combustion of a sulfursource being intended to include such roasting operations or any otherprocess in which a sulfur source is oxidized to produce a sulfurdioxide-containing gas from which sulfuric acid is produced. Sulfurdioxide in the combustion gas is converted to sulfur trioxide in aconverter 103, and gas from the third catalyst stage of the converter isdirected to a heat recovery absorption tower 105. Absorption is carriedout at high temperature in the heat recovery tower, producing sulfuricacid and generating the heat of absorption. The discharge absorptionacid is used for the generation of medium pressure steam in a heatrecovery system boiler 107. Exit gas from the heat recovery tower isdirected back to the converter 103 where residual sulfur dioxide isconverted to sulfur trioxide. Gas from the final converter stage isdirected to a final absorption tower 109 where additional sulfuric acidis produced. Gas leaving the final absorption tower is exhausted fromthe system.

Undried combustion air is drawn into the system through a filter 111 anda compressor 113. The temperature of the combustion air is increased bypassage through an air preheater comprising an indirect heat exchanger115 in which the air is heated by transfer of heat from heat recoverytower discharge absorption acid. In the process illustrated, thecombustion air is passed through one side of the exchanger andabsorption acid through the other side. It will be understood, however,that heating of the combustion air with absorption heat may also beeffected through an intermediary heat transfer fluid, for example apressurized hot water circulating loop. Generally, the acid enteringpreheater 115 is at a temperature of at least about 190° C. and the airis heated to a temperature of at least about 140° C. Preferably, theacid enters the heat exchanger at a temperature of at least about 225°C. and the air is heated to a temperature of at least about 175° C.

Air preheater 115 is preferably a shell and tube exchanger having theacid passing through finned tubes constructed of alloys of the typedescribed hereinbelow as suitable for use in contact with hightemperature sulfuric acid at concentrations in excess of 98.5%.

The heated air is used to burn sulfur or other sulfur source in sulfurburner 101. Thus, the transfer of heat in the air preheater contributesheat to the combustion gas exhausted from burner 101. The gas exitingthe burner is passed through a waste heat recovery unit 117, preferablya steam boiler, where heat is transferred from the combustion gas to aheat transfer fluid. Typically, the combustion gas enters the waste heatboiler at a temperature of about 1160° C. and leaves at a temperatureabove the dew point. Steam is preferably generated at a pressure of atleast about 25 bar gauge, normally in the range of 40 to 60 bar gauge.In the flow sheet illustrated, superheat is imparted to the steamgenerated in the waste heat boiler by passing the steam throughsuperheaters comprising indirect heat exchangers 119 and 121, in whichheat is transferred to the steam from sulfur trioxide-containingconversion gas generated in the converter.

As indicated, combustion air may also be preheated by heat recoveryabsorption acid in a dry gas system. In the latter instance, however,the amount of heat that can be transferred from the heat recoveryabsorption acid to the inlet air (or other oxygen-containing oxidizinggas) may be limited by restrictions on maximum acceptable burnertemperatures. Thus, the improvement relating to the use of absorptionacid for preheating combustion air is especially advantageous in a wetgas process of the type illustrated in FIG. 1. In that system, the coldsuction temperature minimizes the horsepower drawn by blower 113 andalso results in a relatively low temperature for the air discharged fromthe blower. The relatively high mass flow rate of wet air required forcombustion also helps to maintain burner temperature within acceptablelimits, despite the heat energy contributed by preheating the combustionair between blower and burner.

Gas exiting the waste heat boiler 117 enters the first catalyst stage123 of converter 103. Conversion of sulfur dioxide to sulfur trioxide instage 123 generates substantial additional high temperature energy, atleast a portion of which is recovered in superheater 121 in which heatis transferred from the conversion gas to the steam generated in wasteheat boiler 117. Conversion gas exiting superheater 121 enters secondcatalyst stage 125 of the converter, in which additional sulfur dioxideis converted to sulfur trioxide. The hot gas leaving the second catalyststage is cooled by transfer of heat to gas returning to the fourth stageof the converter from heat recovery tower 105. Heat transfer from thesecond stage conversion gas to the returning gas is conducted in aso-called "hot" heat exchanger comprising an indirect heat exchanger127.

Cooled second stage conversion gas exiting hot heat exchanger 127 passesthrough third catalyst stage 129 for further conversion of sulfurdioxide to sulfur trioxide. Heat contained in the gas leaving thirdstage 129 is recovered in superheater 119 by transfer to the steamgenerated in waste heat boiler 117, and in an economizer 131, in whichheat is transferred to boiler feed water for the waste heat boiler.

In a wet gas process, cooling of the gas exiting catalyst stage 129results in vapor phase reaction of sulfur trioxide and water vapor toproduce sulfuric acid vapor. Energy equivalent to a minor portion ofthis heat of formation is recovered in superheater 119 withoutcondensation of the acid. In accordance with the present invention, asubstantial additional fraction of the gas phase heat of formation ofsulfuric acid is recovered at relatively high temperature in theeconomizer. Moreover, the amount of high temperature heat recovered fromthe economizer and/or the amount of intermediate temperature heatrecovered in the heat recovery absorption system is augmented by theinjection of water vapor into the gas stream at a point between burner101 and heat recovery absorption zone 133. Preferably, the water vaporis introduced between catalyst stage 129 and absorption zone 133, morepreferably at a point between catalyst stage 129 and economizer 131.Injection between catalyst stage 129 and superheater 119 is especiallypreferred because it allows for substantial mixing at temperaturessufficient to prevent weak acid condensation upstream of the economizer.Mixing is promoted, inter alia, by the turbulence created by passageover the tubes of the superheater. Thorough mixing upstream of theeconomizer minimizes concentration gradients and any risk of localizedcondensation of weak sulfuric acid in the economizer. The energy contentof low pressure steam injected at a pressure, for example, of 0.2 to 1bar gauge, is upgraded by recovery as intermediate pressure steam fromthe absorption heat recovery boiler 107 or, more preferably, by recoveryas high pressure steam through transfer in condensing economizer 131 tohigh pressure feed water for waste heat boiler 117.

Low pressure steam from a variety of sources may be injected into thegas stream upstream of the condensing economizer. Such sources include,for example, boiler blow down flash, deaerator vent steam, the lowpressure port on a steam turbine for an electrical generator, steamgenerated from low temperature sulfuric acid, heat recovery absorptionsystem steam, and low pressure steam from outside the sulfuric acidplant. A significant benefit from steam injection is realized if steamin which the vapor phase heat of formation is ultimately recovered has apressure substantially higher than the pressure at which steam isintroduced into the gas stream. The greater this difference in pressure,the greater the benefit of the energy quality upgrade that is achievedthrough steam injection. For this reason, it is generally not preferredthat heat recovery system steam, which may typically have a pressure of10 bar gauge, be used as injection steam. As a minimum, on the otherhand, the injected steam must have a pressure slightly in excess of thepressure of the gas stream into which it is injected, i.e., at leastabout 0.2 bar. Generally, the pressure of the injected steam maypractically range from about 0.2 to about 10 bar gauge, preferably about0.3 to about 3 bar gauge, more preferably about 0.3 to about 1 bargauge.

In accordance with the invention, the steam in which the vapor phaseheat of formation of sulfuric acid is ultimately recovered has apressure at least about 2.5 bar higher than the pressure at which steamis introduced into the gas stream. Preferably the difference is at leastabout 8 bar. For applications such as that illustrated in FIG. 1, inwhich the vapor phase heat of formation is transferred to boiler feedwater for high pressure steam, the pressure difference between thatsteam and the injection steam is preferably at least about 25 bar.

It will be understood that, for purposes of the present disclosure, thepressure at which steam (water vapor) is introduced into the gas streammeans the pressure of the steam in the supply line immediately prior toany pressure drop that may be incurred in discharging the steam from thesupply line into the gas stream.

Steam injection is controlled so that the molar ratio of equivalentwater vapor to equivalent sulfur trioxide is maintained at not greaterthan about 1.05. This ensures that the condensation product in thecondensing economizer or heat recovery absorption tower has aconcentration greater than the sulfuric acid azeotrope, which is about99% at 210° C. and about 98.6% at 270° C. These concentrations can behandled using the alloys described herein for use in the tubes of thecondensing economizer. However, localized cooling and high waterconcentration produces weak acid on the steam injection nozzle.Accordingly, this nozzle is preferably constructed of ceramic materialto withstand the aggressive corrosive conditions which prevail.

Economizer 131 comprises an indirect heat exchanger in which heat istransferred to a heat transfer fluid, for example, boiler feed water, asillustrated in the system depicted in FIG. 1. Exchanger 131 comprisesheat transfer wall means, such as the tubes of a shell and tube typeheat exchanger, preferably constructed of an alloy of the type describedin copending application Ser. No. 369,301, as discussed in furtherdetail hereinbelow. In a preferred embodiment of the invention, at leasta portion of the wall means on the gas stream side of the exchanger isat a temperature below the dew point of the gas stream in the exchanger.Thus, sulfuric acid condenses on the heat transfer wall and heat offormation of the condensing acid is transferred to the boiler feedwater.

A condensing economizer may be operated to condense as sulfuric acid asmuch as about 5 to 20% of the sulfur trioxide generated in the firstthree catalyst stages of converter 103. Table 1 shows the heat evolvedwhen sulfur trioxide and water react to form sulfuric acid under variousphase conditions.

                  TABLE 1                                                         ______________________________________                                        Sulfuric Acid Heat of Reaction                                                from Standard Heat of Formation (25° C.)                               No.   Reaction Conditions Heat of Reaction                                    ______________________________________                                        (1)   SO.sub.3 (g) + H.sub.2 O (l) → H.sub.2 SO.sub.4                                            -31.7 kcal/mole                                     (2)   SO.sub.3 (g) + H.sub.2 O (g) → H.sub.2 SO.sub.4                                            -23.3 kcal/mole                                     (3)   SO.sub.3 (g) + H.sub.2 O (g) → H.sub.2 SO.sub.4                                            -42.2 kcal/mole                                     ______________________________________                                    

The gas phase reaction (Equation 2) produces 74% of the heat produced bythe normal liquid phase reaction (Equation 1). Transfer of the heat fromcondensing sulfuric acid to feed water for the waste heat boiler resultsin the ultimate recovery of both the heat of formation and heat ofcondensation of sulfuric acid in the form of high grade energy, i.e.,steam at a pressure of at least about 30 bar gauge, preferably 40 to 60bar gauge.

As indicated by the data of FIG. 2, the conversion of sulfur trioxide tosulfuric acid in the vapor phase increases as the temperature of thevapor phase is lowered. Thus, it is advantageous to lower thetemperature in the condensing economizer 131 to the maximum extentcompatible with effective operation of the heat recovery tower 105. Notonly is the reaction forced to the maximum degree of completion andgeneration of the heat of formation, but the maximum proportion of theheat of formation and condensation of sulfuric acid is recovered in highgrade form by transfer to high pressure boiler feed water for the wasteheat boiler 117. Fortuitously, it has been discovered that economizer131 can be operated to extract a maximum amount of the vapor phaseenergy of formation of sulfuric acid without the necessity for closecontrol of the fluid flow rates or wall temperatures within theeconomizer. As illustrated in FIG. 3, the concentration of acid in thecondensate varies only very gradually with the water/sulfur trioxideratio in the gas phase, and consequently does not vary significantlywith either the temperature to which the gas is cooled or the walltemperature of the heat exchanger. Thus, it is not necessary to closelycontrol the operation of the condensing economizer to avoid corrosiveconditions therein. Consequently, variations in inlet air humidity, orexcursions in sulfur flow rate or steam injection rate, do notmaterially affect the concentration of the acid condensing on the tubewalls of the condensing economizer. FIG. 3 shows that as much as 140% ofthe stoichiometric amount of water vapor may be added by injectionwithout reducing the concentration of the condensing acid below 98%.

It has been found that energy equivalent to about 40 to 70%, mosttypically about 60%, of the heat of formation of sulfuric acid vapor isrecovered by cooling the gas stream between catalyst stage 129 andabsorption zone 133. Although the use of superheater 119 is advantageousfor imparting superheat to the steam generated in waste heat boiler 117,40 to 70% recovery of the vapor phase heat of formation may be achievedin economizer 131 alone. Where both superheater 119 and condensingeconomizer 131 are used, about 70% to about 90%, generally about threefourths, of the recovered heat of formation is transferred in thecondensing economizer. Typically, the gas stream entering the condensingeconomizer has a temperature in the range of between 470° and about 320°C. and an H₂ O/SO₃ mole ratio of between about 0.2 and about 1.05. Thegas stream leaving the condensing economizer has a temperature in therange of about 240° to about 300° C. Boiler feed water typically entersthe economizer at a temperature of between about 110° and about 180° C.

It will be understood that a substantial portion of the vapor phase heatof formation of sulfuric acid can be extracted without condensation ineconomizer 131. In some circumstances, it may be desirable to operatethe economizer under conditions which preclude condensation since thisallows the economizer to be constructed of carbon steel instead of aFe/Cr or Fe/Cr/Ni alloy. Thus, for example, recovery of a substantialfraction of the heat of formation may be achieved without condensationby transferring heat from the gas stream to boiler feed water in aco-current heat exchanger. However, in most instances it is preferredthat an alloy exchanger be used and that the tube walls be operated at atemperature low enough to cause condensation thereon, though not so lowas to cause nucleation and mist formation within the bulk gas stream. Bysuch means a high portion of the heat of formation, and an appreciablefraction of the heat of condensation, of sulfuric acid is recovered inthe form of high pressure steam.

The wet gas stream leaving economizer 131 is directed to the heatrecovery tower 105 where it is contacted countercurrently with sulfuricacid in a heat recovery absorption zone 133 within the tower. Zone 133comprises means, such as packing, for promoting mass transfer and heattransfer between the gas and liquid phases within the zone. The inletgas to the absorption zone contains sulfur trioxide, water vapor, andsulfuric acid vapor. Contact of the gas with liquid sulfuric acid causesabsorption of sulfur trioxide, condensation and absorption of watervapor, and condensation and absorption of sulfuric acid vapor in theliquid sulfuric acid stream. It will be understood that, within thecontext of this disclosure, the terms "heat of absorption" and "energyof absorption" include all of these various heat effects. Such may alsoinclude energy of formation of sulfuric acid in the vapor phase that hasnot been recovered in condensing economizer 131.

By use of hot acid for absorption in zone 133, two important goals arerealized. First, the heat of absorption is generated at relatively hightemperature which allows subsequent recovery of this energy at hightemperature. Additionally, the use of high temperature acid avoids shockcooling of the gas stream and consequently minimizes the formation ofacid mist in the wet gas. FIG. 4 shows the effect of heat recovery towerdischarge acid temperature on mist formation in gas entering at 300° C.and two different H₂ O/SO₃ ratios. Preferably, the temperature of theacid at the exit of zone 133 is no cooler than about 40° C. below, morepreferably no more than 20° C. below, the dew point of the inlet gas.Surprisingly, it has been discovered that the gas can be at atemperature below 300° C. as it enters the absorption zone, therebyallowing recovery of the maximum amount of the energy of vapor phaseformation and condensation of sulfuric acid in the form of high pressuresteam as a result of the transfer of this heat to the high pressureboiler feed water for waste heat boiler 117.

Acid is discharged from the heat recovery stage at a temperature of atleast 190° C., generally between about 190° and about 250° C.Preferably, the exit acid temperature should be in the range of betweenabout 210° and about 250° C., the optimum being near the gas dew point.The temperature of the incoming gas is typically in the range of about240° to about 300° C.

As shown in FIG. 1, the gas flows upward through the packed absorptionstage (sometimes referred to hereinafter as the "heat recovery zone" or"heat recovery stage"). It will be understood that other gas liquidcontacting devices such as a countercurrent tray tower or a co-currentventuri absorber can be used in lieu of a packed tower.

Sulfuric acid is delivered to the top of the absorption zone 133 at atemperature preferably between about 170° and about 220° C., and aconcentration broadly in the range of between about 98.5% and about99.5%. However, because injection of steam into the gas leaving catalyststage 129 causes the equivalent water to sulfur trioxide mole ratio inthe gas phase entering zone 133 to be in the range of about 0.2 to 1.05,preferably 0.7 to 1.0, it is preferred that the strength of the acidentering the tower be in the range of about 99% to about 99.5%, thisconcentration remaining essentially constant throughout the absorptionzone.

Although the absorption stage is operated at elevated temperatures, atleast about 90% of the equivalent sulfur trioxide in the inlet gasstream is absorbed in the heat recovery stage. For purposes of thisdisclosure "equivalent sulfur trioxide" is defined as the molar sum ofthe sulfur trioxide and sulfuric acid in the gas phase. Similarly,"equivalent water vapor" is the molar sum of water vapor and sulfuricacid in the gas phase.

Sulfuric acid leaving the absorption zone 133 flows to a circulatingpump 135, at the discharge of which the acid stream is divided into twostreams, one containing a major proportion of the acid. This majorstream is conducted to indirect heat exchanger 107 where the energy ofabsorption is recovered by transfer of heat to another fluid. The minorportion of the absorption acid discharge stream is directed to preheater115 for transfer of heat to air that is used for combustion of thesulfur source. Preferably, as illustrated in FIG. 1, heat exchanger 107comprises a boiler for the generation of medium pressure steam, forexample, steam having a gauge pressure between approximately 1.5 and 20bar and normally between about 3 and about 12 bar. In a particularlypreferred mode of operation, the acid leaving the heat recoveryabsorption zone is maintained at a temperature greater than 200° C., andsteam is generated in heat recovery boiler 107 at a pressure of 3.0 bargauge or greater, preferably greater than 10 bar gauge. Steam generatedin heat exchanger 107 by transfer of the absorption heat may be used ina variety of applications.

Any portion of the vapor phase heat of formation of sulfuric acid notrecovered by transfer to high pressure steam in superheater 119 andtransfer to high pressure boiler feed water in economizer 131 istransferred in heat recovery zone 133 to the circulating absorptionacid. Most if not all of the heat of condensation of the acid formed inthe vapor phase is also transferred to the absorption acid. This heatenergy is ultimately recovered in the form of medium pressure, e.g., 3.0bar to 10 bar, steam in heat recovery system boiler 107.

Acid streams returning from heat recovery system boiler 107 and airpreheater 115 are combined and at least a portion of the combined streamis recirculated to the heat recovery tower at a point above absorptionstage 133. To maintain a constant volume of acid in the circuit anotherportion of the combined return acid stream is removed from the circuitas overflow acid through line 137. Additional heat is recovered from theoverflow acid by passing it in series through indirect heat exchangers139 and 141 which comprise preheaters for feed water to heat recoverysystem boiler 107 and deaerator 165, respectively. Acid leavingpreheater 141 is delivered to a pump tank 143 containing circulatingacid for final absorption tower 109.

Trim control of the concentration of the heat recovery absorption systemcirculating acid is provided by dilution of the returning acid in a amixing stage 145. Mixing water may be added in vapor form, therebyproviding for recovery of the heat of vaporization at relatively hightemperature in the heat recovery boiler 107.

As a result of the high temperature operation of the heat recoverystage, the gas stream exiting the top of this stage is relatively hotand is in contact with hot acid. This in turn results in strippingsulfuric acid from the acid stream into the gas stream. Although theabsorption efficiency of the heat recovery stage is at least about 90%,high temperature operation of the heat recovery stage also results insome unabsorbed SO₃ passing through that stage. Gas exiting the top ofthe heat recovery stage is therefore directed to a condensing stage 147for absorption of residual sulfur trioxide and condensation of sulfuricacid vapor. Condensing stage 147 contains means for promoting gas/liquidcontact and mass transfer and heat transfer therebetween. Preferably,this stage comprises a countercurrent packed section. Relatively coolacid having a concentration of about 98.5% is fed to the top of thisstage and gas leaving the heat recovery stage at a temperature of about170° to about 230° C. enters the bottom of the condensing stage.

At the gas exit from the condensing stage, it is preferred that thetemperature of the acid entering be below about 120° C., most preferablybetween about 60° and 80° C. On passage through the condensing stage thegas stream is typically cooled to a temperature in the range of betweenabout 75° and 140° C. normally between about 80° and about 120° C.

The acid flow rate in the condensing stage is maintained at a rate lowenough that the acid leaves the stage at a temperature which approachesthe temperature of the acid entering the heat recovery stage. Thus, theweight basis flow rate of the absorption stage acid is at least abovefour times, preferably between about four and about twenty times, thatof the condensing stage acid stream.

Gas leaving condensing stage 147 passes through a mist eliminator 149within tower 105 and then exits the tower returning to the converter viaan indirect heat exchanger comprising a so-called cold heat exchanger151 and hot heat exchanger 127. The return gas is thus heated to atemperature appropriate for further conversion of sulfur dioxide tosulfur trioxide in final conversion stage 153.

In cold heat exchanger 151, the gas flowing to final conversion stage153 is preheated by transfer of heat from the gas leaving that samestage, while in hot heat exchanger 127 the return gas is preheated bytransfer of heat from gas leaving second catalyst stage 125.

Fourth stage conversion gas leaving cold heat exchanger 151 is directedto final absorption tower 109 through an indirect heat exchangercomprising an economizer 155 where heat is transferred from theconversion gas to boiler feed water for the waste heat boiler 117.

Absorption of residual sulfur trioxide is carried out in finalabsorption tower 109 by countercurrent flow of sulfuric acid and the gasover a packed absorption zone 157. Acid is circulated over the towerfrom pump tank 143 via a circulating pump 159 and an acid cooler 161.Dilution water for the final absorption tower is added at pump tank 143.Acid produced in the final absorption step is removed from the processvia a cooler 163.

In the process of FIG. 1, boiler feed water passes through finalabsorption acid cooler 161, preheater 141, and a deaerator 165. Feedwater leaving deaerator 165 is divided. One portion flows to a highpressure boiler feed water pump 167, the other to a heat recovery boilerfeed water pump 169. Water from pump 169 is fed to the heat recoverysystem via preheater 139. Water from pump 167 is fed to waste heatboiler 117 via economizers 155 and 131.

The process of the invention is uniquely capable of recovering not onlyprocess heat but latent heat of the humidity of incoming air in the formof high grade energy. In the latter connection, it may further be notedthat a wet scrubber may be substituted for air filter 111, and thehumidity picked up in the scrubber ultimately recovered in the form ofhigh temperature energy. Because the air leaving the scrubber isessentially at its wet bulb temperature, a significant increment ofenergy may be picked up at this point.

Wet sulfur trioxide-containing gas can be handled in carbon steelequipment provided that the gas temperature is kept above the dew point.In the preferred embodiments of the present invention, the dew point isgenerally high, so that carbon steel equipment is suitable only for thewaste heat boiler, high temperature superheaters, the gas heatexchangers 127 and 151, and the economizer 155. Equipment operated belowthe dew point or otherwise in contact with hot liquid sulfuric acid,such as the condensing economizer, heat recovery system boiler, heatrecovery system boiler feed water preheaters, and air preheater, hasheat transfer surfaces constructed of alloys or other corrosionresistant materials. There are a number of stainless steel and nickelalloys that can be used in high temperature strong sulfuric acidservice. Alloy performance can be characterized by a corrosion index(CI) which is defined in terms of alloy composition by the followingrelationship:

    CI=0.4[Cr]-0.05[Ni]-0.1[Mo]-0.1[Ni]×[Mo]

Where:

[Cr]=Weight percent chromium in the alloy

[Ni]=Weight percent nickel in the alloy

[Mo]=Weight percent molybdenum in the alloy.

Alloys which work best in high temperature strong sulfuric acid servicehave been found to have a corrosion index greater than 7, preferablygreater than 8.

The alloys most likely to exhibit low corrosion rates are those with thehighest corrosion index. As indicated by the corrosion index formula,high chromium is desirable, and it is preferable to avoid alloys whichhave both high nickel and high molybdenum. However, alloys which containhigh nickel and very low molybdenum, or low nickel and moderate amountsof molybdenum are usually found to be acceptable. Particular alloysfound suitable for use in contact with liquid phase sulfuric acid athigh temperature include those having UNS designations S30403, S30908,S31008, S44627, S32304, and S44800.

                                      TABLE 2                                     __________________________________________________________________________    Alloy                                                                              % by weight - Components of Alloy                                        UNS No.                                                                            C    Mn  Si  Cr    Ni   Others                                           __________________________________________________________________________    S30403                                                                             0.03 2.00                                                                              1.00                                                                              18.0-20.0                                                                            8.0-12.0                                                                          0.045 P; 0.03 S; 0.10 N                          S30908                                                                             0.08 2.00                                                                              1.00                                                                              22.0-24.0                                                                           12.0-15.0                                                                          0.045 P; 0.03 S                                  S31008                                                                             0.08 2.00                                                                              1.50                                                                              24.0-26.0                                                                           19.0-22.0                                                                          0.045 P; 0.03 S                                  S44627                                                                             0.01 0.40                                                                              0.40                                                                              25.0-27.5                                                                           0.50 0.02 P; 0.02 S; 0.75-1.5 Mo; 0.015 N; 0.2                                     Cu; 0.5(Ni + Cu); 0.05-0.20 Nb                   S32304                                                                              0.030                                                                             2.50                                                                              1.0 21.5-24.5                                                                           3.0-5.5                                                                            0.040 P; 0.040 S; 0.05-0.2 N; 0.05-0.6 Mo;                                    0.05-0.6 Cu                                      S44800                                                                              0.010                                                                             0.30                                                                              0.20                                                                              28.0-30.0                                                                           2.0-2.5                                                                            0.025 P; 0.02 S; 3.5-4.2 Mo; 0.15 Cu;            The balance in each instance is essentially iron.                             __________________________________________________________________________

For safe and corrosion free start-ups and shut downs two extra ducts maybe added to the process flow diagram of FIG. 1. These include a recycleduct from the final absorbing tower outlet to the blower inlet and aheat recovery system bypass from the third catalyst stage outlet to thefourth catalyst stage outlet. In this operation, initial heat up isaccording to conventional practice in which fuel is burned in the sulfurburner and the combustion products of the fuel vented after the wasteheat boiler 117.

For cooling and purging of the plant, dry air is recycled through theentire plant. Residual sulfur trioxide and heat are removed in the finaltower and its acid cooler. Also during start-up, liquid dilution wateris preferably used. This lowers the dew point by about 60° C. in the gasentering superheater 119. Steam injection is not initiated until allheat exchange surfaces have reached steady state operating temperature.

EXAMPLE 1

Set forth in Table 2 are typical temperatures of the various processstreams in a wet gas sulfur burning sulfuric acid manufacturing processoperated in accordance with the flow sheet of FIG. 1. Also set forth aretemperatures and pressures of steam generated in the process.

                  TABLE 2                                                         ______________________________________                                               Temperature         Pressure Temperature                               Stream #                                                                             (°C.)                                                                             Stream # (Bar Gauge)                                                                            (°C.)                              ______________________________________                                         1      38        56                138                                        2      38        57                162                                        3      87        58                266                                        4     192        59                138                                        5     1161       60                184                                        6     420        61                 32                                        7     597        62                227                                        8     440        63                227                                        9     528        64                193                                       10     440        65                227                                       11     469        66                200                                       12     454        67                198                                       13     367        68                198                                       14     271        69                199                                       15      83        70                198                                       16     307        71                179                                       17     425        72                 93                                       18     447        73                 91                                       19     222        74                 93                                       20     147        75                 70                                       21      77        76                 77                                       50     132        77                 43                                       51      44        78       0.4      109                                       52      86        79       62       279                                       53     131        80       60       351                                       54     131        81       59       482                                       55     138        82       11       187                                       ______________________________________                                    

EXAMPLE 2

A pilot plant was operated using steam injection to supply dilutionwater. Over 1000 hours of operation were logged in accordance with thisprocess flow scheme. Corrosion coupons were inserted in certain processlocations and measurements made of corrosion rates. Set forth in Table 3are the corrosion rates for coupons of various materials at a pointapproximately 1.5 meters downstream of the steam injection nozzle. Thisdata was taken with 100% of dilution water for the heat recovery towerbeing supplied by steam injection, and with the metal surfaces beingbelow the dew point of the gas.

                  TABLE 3                                                         ______________________________________                                        Corrosion Rates Measured in Pilot Plant Downstream of Equi-                   molar Steam Injection Average Metal Temperature = 235° C.              Alloy      Corrosion Rate (mm/a)                                              ______________________________________                                        S30403     0.020                                                              S30908     0.013                                                              S31008     0.008                                                              ______________________________________                                    

What is claimed is:
 1. In a process for the manufacture of sulfuric acid, comprising combustion of a source of sulfur with an oxygen-containing gas in a burner to produce a combustion gas stream comprising sulfur dioxide and oxygen, passage of the gas stream through a plurality of catalyst stages for progressive conversion of sulfur dioxide to sulfur trioxide, recovery of heat in useful form by cooling the gas stream exiting each of said catalyst stages, passage of the cooled gas stream from one of said stages through an absorption zone where the gas stream is contacted with sulfuric acid for removal of sulfur trioxide from the gas phase, and return of the gas stream from said zone to a further stage of said plurality of stages, the improvement which comprises:introducing water vapor into the gas stream at a point between said burner and the said absorption zone, at least a portion of the water vapor reacting with sulfur trioxide in the gas phase to produce sulfuric acid and thereby generate the heat of formation of sulfuric acid in the gas phase and recovering heat energy from the vapor phase heat of formation of sulfuric acid by transfer of heat in an indirect heat exchanger from said gas stream to steam having a pressure at least about 8 bar higher than the pressure of said water vapor as introduced into the gas stream or to feed water from which said steam is generated wherein said indirect heat exchanger is located upstream of said absorption zone.
 2. An improved process as set forth in claim 1 wherein said water vapor is introduced into said gas stream between said one stage and said indirect heat exchanger.
 3. An improved process as set forth in claim 1 wherein said steam is generated at a pressure of at least about 25 bar gauge.
 4. An improved process as set forth in claim 3 wherein the pressure of said steam is at least about 15 bar higher than the pressure at which water vapor is introduced into said gas stream.
 5. An improved process as set forth in claim 4 wherein the pressure of said steam is at least about 40 bar higher than the pressure at which water vapor is introduced into said gas stream.
 6. An improved process as set forth in claim 1 wherein said indirect heat exchanger comprises an economizer in which heat is transferred from said gas stream to said feed water.
 7. An improved process as set forth in claim 6 wherein said economizer comprises heat transfer wall means between said gas stream and said feed water, at least a portion of said wall means on the gas stream side thereof being at a temperature below the dew point of the gas stream entering the economizer.
 8. An improved process as set forth in claim 6 wherein said steam is superheated by transfer of heat from said gas stream in an indirect heat exchanger that is between said one stage and said economizer with respect to the flow of said gas.
 9. An improved process as set forth in claim 8 wherein between about 40% and about 70% of the heat of formation of sulfuric acid in said gas stream is recovered by transfer of heat to said steam in said superheater and said economizer.
 10. An improved process as set forth in claim 8 wherein to between about 70% and about 90% of the gas phase heat formation of sulfuric acid that is transferred to said steam is transferred in said economizer.
 11. An improved process as set forth in claim 7 wherein the material of construction of said heat transfer wall means comprises an iron/chromium, nickel/chromium, or iron/chromium/nickel alloy having a composition which has a corrosion index, CI, ≧7 corresponding to the algorithm:

    CI=0.4(Cr)-0.05(Ni)-0.1(Mo)-0.1(Ni)×(Mo)

where: (Cr)=the % by weight of chromium in the alloy (Ni)=the % by weight of nickel in the alloy (Mo)=the % by weight of molybdenum in the alloy.
 12. An improved process as set forth in claim 11 wherein said material of construction is selected from the group consisting of UNS alloy S30403 containing between about 18.0 and about 20.0% by weight chromium, between about 8.0 and about 12.0% by weight nickel, up to about 0.03% by weight carbon, up to about 2.00% by weight manganese, up to about 1.00% by weight silicon, up to about 0.045% by weight phosphorus, up to about 0.03% by weight sulfur, and up to about 0.10% by weight nitrogen, UNS alloy S30908 containing between about 22.0 and about 24.0% by weight chromium, between about 12.0 and about 15.0% by weight nickel, up to about 0.08% by weight carbon, up to about 2.00% by weight manganese, up to about 1.00% by weight silicon, up to about 0.045% by weight phosphorus, and up to about 0.03% by weight sulfur, UNS alloy S31008 containing between about 24.0 and about 26.0% by weight chromium, between about 19.0 and about 22.0% by weight nickel, up to about 0.08% by weight carbon, up to about 2.00% by weight manganese, up to about 1.50% by weight silicon, up to about 0.045% by weight phosphorus, and up to about 0.03% by weight sulfur, UNS alloy S44627 containing between about 25.0 and about 27.5% by weight chromium, up to about 0.50% by weight nickel, up to about 0.01% by weight carbon, up to about 0.40% by weight manganese, up to about 0.40% by weight silicon, up to about 0.02% by weight phosphorus, up to about 0.02% by weight sulfur, between about 0.75% and about 1.5% by weight molybdenum, up to about 0.015% by weight nitrogen, up to about 0.2% by weight copper, up to about 0.5% by weight nickel plus copper and between about 0.05% and about 0.20% by weight niobium, UNS alloy S32304 containing between about 21.5% and about 24.5% by weight chromium, between about 3.0 and about 5.5% by weight nickel between about 0.05 and about 0.6% by weight molybdenum, between about 0.05 and about 0.6% by weight copper, between about 0.05 and about 0.2% by weight nitrogen, up to about 0.030% by weight carbon, up to about 2.5% by weight manganese, up to about 1.0% by weight silicon, up to about 0.04% by weight phosphorus, and up to about 0.04% by weight sulfur, and UNS alloy S44800 containing between about 2.0 and about 2.5% by weight nickel, up to about 0.010% by weight carbon, up to about 0.30% by weight manganese, up to about 0.20% by weight silicon, up to about 0.025% by weight phosphorus, up to about 0.02% by weight sulfur, between about 2.5 and about 4.2% by weight molybdenum, and up to about 0.15% by weight copper.
 13. An improved process as set forth in claim 1 wherein the pressure of said water vapor as introduced into said gas stream is no greater than about 10 bar.
 14. An improved process as set forth in claim 13 wherein the pressure of said water vapor as introduced into said gas stream is between about 0.2 and about 3 bar.
 15. An improved process as set forth in claim 1 wherein the pressure of said water vapor as introduced into said gas stream is between about 0.2 and about 1 bar.
 16. An improved process as set forth in claim 1 wherein the exit gas from said heat recovery absorption stage is contacted with sulfuric acid in a condensing stage for absorption of residual sulfur trioxide and condensation of sulfuric acid vapor, said contact in said condensing stage being carried out before said exit gas is either exhausted from the process or catalytically oxidized to produce additional sulfur trioxide.
 17. An improved process as set forth in claim 1 wherein the dew point of the gas stream entering the heat recovery stage does not exceed the temperature of the discharge sulfuric acid stream by more than about 40° C.
 18. An improved process as set forth in claim 1 wherein said combustion gas is a wet gas and the gas exiting said one catalyst stage is a wet conversion gas. 